1. Field of the Invention
The present invention relates to solid-state lasers. More specifically, the present invention pertains to laser diodes and coupling of laser diodes to multimode optical fibers.
2. Description of Related Art
It is often necessary to couple a laser diode to a multimode optical fiber. The standard procedure is to butt couple the laser diode to the optical fiber, placing the optical fiber as close as possible to the emitting face of the laser diode. The output from a typical laser diode has very high angular divergence in the direction perpendicular to the diode junction, having a typical numerical aperture (NA) of 0.4 to 0.6. The numerical aperture is the sine of the half angle of the diode emission. The NA in the direction parallel to the junction is much smaller, typically 3 times less.
In order to couple efficiently to a laser diode, a multimode optical fiber must have an acceptance angle or NA comparable to the NA of the laser diode. A multimode optical fiber ideal for coupling to a laser diode would have an NA of 0.3 or above. Such a multimode optical fiber is somewhat difficult to design since it requires a cladding having a very low index of refraction. Moreover, since the energy emitted from the laser diode diverges rapidly, the end of the optical fiber must be placed very close to the diode facet to which it is to be coupled. Accurately achieving such placement requires very tight mechanical tolerances and can lead to practical fabrication problems due to optical feedback to the laser diode.
An object of the present invention is to provide a convenient means of coupling a low NA multimode optical fiber to a laser diode. The resulting low NA fiber output can have high brightness, where brightness is defined as W/cm.sup.2 /unit solid angle.